DACs -- a technical discussion

[tony]

Nigel there is a dcs stack for sale in Cloneys http://cloneyaudio.com/explore/ex-demosecondhand-deals/
Just be careful if you buy it as blowing it up will be pricey unless Dave is going to put it back together again.

[nige2000]

On the subject of an FPGA based project here, I've been asked about this a few times and regrettably I just wouldnt have the time to take on such a task. I could get a few days here and there but then would be pulled off with work for weeks on end.

Ok I understand :(
Take it its not a weekend type project then?

Would have thought such a device would have great potential

[nige2000]

Nigel there is a dcs stack for sale in Cloneys http://cloneyaudio.com/explore/ex-demosecondhand-deals/
Just be careful if you buy it as blowing it up will be pricey unless Dave is going to put it back together again.

Why do you think i try to build cheap?

[DaveF]

Take it its not a weekend type project then?

Not really no. Depends on what you want to do in the FPGA? Also do you require a custom PCB or will it come on an pre-fabed evaluation board? All this has implications for the FPGA in terms of pinout available and other issues.

Would have thought such a device would have great potential

Indeed it would. And with an FPGA you can upgrade it through a software mechanism allowing you to constantly upgrade improve, refine and add features. This how dCS, Chord, Hugo and no doubt Devialet do it.

[tony]

Nigel there is a dcs stack for sale in Cloneys http://cloneyaudio.com/explore/ex-demosecondhand-deals/
Just be careful if you buy it as blowing it up will be pricey unless Dave is going to put it back together again.

Why do you think i try to build cheap?

Of course not but check out the secondhand price of that DCS stuff secondhand (original cost I think might be circa €20k) if you plan attacking after purchase I thought best to warn you given track record

[jrling]

I dont know the details of the Chord DAC64 but an FPGA sitting before the DAC is most likely to be doing some type of digital filtering or a choice of filtering as well the possibility of upsampling/interpolation etc. An FPGA is a pretty powerful device capable of doing a lot with audio. Any idea what FPGA it is? (Just to get an idea of the size, logic gates and mem internally)

dCS use Xilinx FPGA's in their DAC's. Well the older Elgar DACs anyway.

Hi Dave - good to hear from you.
Just to confirm that Chord's DAC64 MkII is implementing the DAC in the FPGA and in later models they took it to greater highs with high resolution DAC in the FPGA. It is Xilinx but I don't know the exact model. There is another Xilinx in there which I resume is taking care of the RAM buffering and reclocking before passing the signal to the DAC FPGA.

As you know I am one of those who encouraged you to be involved in an FPGA project. I completely understand why you would decline, as I can see that it would be a major project and requiring a load of testing, refinement and prototyping.

Shame noone else is offering, as Nige is right, it would be the way to go for ultimate SQ. IMHO. The Chord Hugo is rather proving that point, for those who can afford it.

[DaveF]

Hi Dave - good to hear from you.
Just to confirm that Chord's DAC64 MkII is implementing the DAC in the FPGA and in later models they took it to greater highs with high resolution DAC in the FPGA. It is Xilinx but I don't know the exact model. There is another Xilinx in there which I resume is taking care of the RAM buffering and reclocking before passing the signal to the DAC FPGA.

But what DAC chip are they using? An FPGA is not a DAC. It operates purely in the digital domain. Therefore there must another DAC chip following the FPGA be it an inhouse developed one or off the shelf. Are there schematics or closeup pics of the PCB available?

[Aleg]

DaveF

This is a close up of the Chord Hugo so you can see the model of the Spartan 6





And the DAC chip used, two BurrBrown PCM2706 one for each channel


In this review there are more clos-up pictures and detailed specs:
http://soundnews.ro/2014/03/13/chord-hugo-review/


And the block diagram of the Hugo is found here:
http://www.phileweb.com/interview/artic ... =221&row=7

Cheers

Aleg

[DaveF]

Some nice clever stuff going on in the Spartan 6 design. Filtering, upsampling, interpolation, noise shaping etc. Definitely the domain of an FPGA.
Its a small Spartan 6 though, the LX9 which is the second smallest in that product line. Not many DSP blocks internally but enough it seems
for this job.
The DAC itself appears to be a standard stereo audio DAC from TI with USB/I2S interface.
So the 'secret sauce' of this Hugo DAC is in the FPGA I would think.

Perhaps this should be moved out of the MQN thread? :-)

[jkeny]

The PCM2706 is not used as a DAC - it is used as a USB receiver for connecting to iPhone, etc
Here's what Rob Watts says

The PCM2706 is just used for the SD USB, it most categorically is not the DAC. The DAC is a pulse array DAC, which uses discrete flip-flops for the analogue OP.